The present invention relates to field of DC to AC inverters, and in particular, to transformerless inverters for converting a DC voltage to two-phase AC.
Two-phase AC electrical systems are commonly used and required in residential and industrial applications to power different types of loads. A two-phase AC system consists of two AC voltages 180 degrees out of phase with respect of a common neutral to produce a system with two different output voltages: one between each phase and neutral and one between the two phases. A common residential 240/120 VAC system is typical of such a two-phase system.
Two-phase systems are usually produced from a single-phase source connected to a transformer with center tap output, which gives the two required AC sources. The single-phase source may come from any power generation equipment including a conventional power electronics inverter as illustrated in FIG. 1. In power generation systems in the range of a few kilowatts, it is desirable to reduce the size and weight of the unit. An output transformer operating at low frequencies represents a very bulky piece of equipment, and a transformerless system would clearly be advantageous in this respect.
In a one-leg inverter, two high-power switching transistors are used. One switch is connected to the positive rail of the DC source and tied to a common node. The other switch is connected to the negative rail and tied to the common node. The AC power is extracted from the common node with respect to a neutral point at the middle voltage of the DC bus. This switch configuration is the fundamental block used in conventional inverters and is well known in the art.
It is known to use three one-leg inverter configurations to transform DC power to three-phase AC power. A three-phase bridge essentially consists of three one-leg inverters which are synchronized so that each of the three phases are separated by 120xc2x0.
Theoretically, a two-phase source may be generated from two one-leg inverter modules fed by a DC-link with a mid-potential connection as shown in FIG. 2. In this case, each of the inverter legs is switched to modulate the desired waveforms with respect of the neutral midpoint. However, a problem with this circuit is that any DC component introduced by the load will result in an unbalance between the upper and the lower half of the DC-link and eventually may result in incorrect system operation. Furthermore the mid-potential point from the DC-link is not always accessible.
Therefore, there is a need in the art for a two-phase DC to AC inverter which mitigates the difficulties known or presented in the prior art.
In general terms, the invention comprises a transformerless DC to two-phase AC inverter. In one aspect, the invention comprises an apparatus for converting DC power to two-phase AC power, comprising:
(a)three one-leg switch mode inverters wherein a first leg produces VPhase1, a second leg produces Vneutral, and a third leg produces VPhase2 and wherein VPhase1 is out of phase with VPhase2;
(b)a controller/driver circuit having a switching waveform generator and first and second sinusoidal reference waveform generators, the sinusoidal waveforms generated at the desired AC output frequency, and the switching signal generated at a higher frequency;
(c)means for comparing the switching waveform with the first reference waveform to produce a first pulse width modulated signal which drives the first leg;
(d)means for comparing the switching waveform with the second reference waveform to produce a second pulse width modulated signal which drives the third leg;
(e)means for determining the slope of the switching waveform which drives the second leg with a 50% duty cycle.
In another aspect, the invention may comprise a method of converting DC power to two-phase AC power utilizing three one-leg switch mode inverters wherein the first leg produces VPhase1, the third leg produces VPhase2 and the second leg produces VNeutral and wherein VPhase1 is out of phase with VPhase2, comprising the steps of:
(a)generating a first reference sinusoidal waveform and a second reference sinusoidal waveform which is out of phase with the first reference waveform,
(b)generating a switching waveform having a higher frequency than the first and second reference waveforms;
(c)comparing the switching waveform with the first reference waveform to produce a first pulse width modulated signal which drives the first leg;
(d)comparing the switching waveform with the second reference waveform to produce a second pulse width modulated signal which drives the third leg;
(e)determining the slope of the switching waveform which drives the second leg with a 50% duty cycle; and
(f)filtering the switching waveform from VPhase1 VNeutral and from VPhase2 VNeutral.